1. Field of the Invention
The field of the invention is that of devices for detecting objects concealed on human subjects. These devices are more particularly dedicated to the surveillance and protection of airport areas and transport airplanes, but can also be positioned at the entrance of protected buildings or controlled access areas such as boat or train embarkation platforms.
2. Description of the Prior Art
To ensure the safety of the passengers in aircraft, cargo hold luggage and hand baggage is checked by X-ray imaging systems. The passengers themselves pass only through a metal-detector gate. Now, it is essential to detect on a passenger non-metallic objects that present a real danger such as explosives or ceramic weapons.
To overcome this security omission, some airports have put in place, experimentally, X-ray scanners for the passengers themselves. However, the use of X-rays for a non-medical purpose is prohibited in a large number of countries and in particular in most European states. In practice, this technique includes a real danger for the human being when exposed to regular use.
To overcome the drawbacks of using X-rays, it is possible to take an image of the human body in the field of millimetric electromagnetic waves. In practice, the objects or dangerous materials that we are trying to detect reflect the waves in a manner that is very different from that of the human body. This means they can easily be detected. This imaging can be done either passively, or actively. The passive technique consists in taking an image directly of the body without illuminating it with a particular millimetric source. In contrast to this, the active technique can be used to take an image by illuminating the body, for example, with a known millimetric beam with a precise wavelength.
These techniques have a number of drawbacks. They are complex to implement. Some use costly mirror-deflection systems. Systematically installing them in an airport therefore involves considerable investments. Also, the techniques consisting in taking the image of the human body come up against an ethical problem. In practice, since clothes are not very dense and are unconstructed, they are transparent to the millimetric radiation and, consequently, the subject appears nude on the millimetric image. Now, the passenger will not accept being analyzed nude by an operator.
To overcome these drawbacks, it is possible to simplify the systems by using small portable devices which make it possible to scan the observed body without taking images. Normally, these devices take a measurement of the power reflected by the human body as a function of the frequency of the transmitted wave. This is used to determine the significant electromagnetic resonances that reveal the presence of dielectric objects. It is also possible to have the detection based on measurements of polarization or ellipsometry of the waves reflected by the human body, the polarization characteristics depending on the nature of the illuminated body. One of the main drawbacks of these single-detector systems is that the measurement is a spot or almost spot measurement. Consequently, checking a subject can take some time. These devices are ill-suited to quickly checking a large number of passengers. It can also be difficult to determine very accurately the position or the size of a suspect object.